The minimum nominal cover to reinforcement for R.C.C. footings as per IS: 456-2000 shall be
50 mm
Understanding Minimum Nominal Cover for R.C.C. Footings
The question asks about the minimum nominal cover required for reinforcement in R.C.C. footings as specified by the Indian Standard code IS: 456-2000 for Plain and Reinforced Concrete.
Nominal cover to reinforcement is the distance from the surface of the concrete to the nearest surface of the reinforcing bar. This cover is crucial for protecting the steel reinforcement from corrosion and providing adequate fire resistance and bond strength.
For structural elements like footings, which are typically in contact with or buried in the ground, the environmental conditions are considered severe or very severe. These conditions necessitate a larger nominal cover to ensure the durability of the structure over its design life.
IS: 456-2000 Provisions for Nominal Cover
IS: 456-2000 provides guidelines for the minimum nominal cover based on the exposure condition and the type of structural element. Table 16 of the code specifies these minimum covers. For footings, which are foundation elements, the code specifies a particular minimum cover.
According to Clause 26.4.2.2 of IS: 456-2000, for footings, the minimum cover to the reinforcement shall be 50 mm. This requirement is specifically for footings regardless of the exposure condition mentioned in Table 16 for other elements, although footings are generally subjected to severe environmental conditions.
Analyzing the Options
Let's look at the given options and compare them with the code provision:
- 15 mm: This cover is generally inadequate for footings and is typically used for elements like slabs or walls in mild exposure conditions.
- 5 mm: This value is extremely low and is not a valid nominal cover for any structural reinforcement according to IS: 456.
- 20 mm: This cover might be suitable for beams or columns in mild exposure conditions, but it is insufficient for footings.
- 50 mm: This value matches the specific requirement for footings as per IS: 456-2000.
Based on the provisions of IS: 456-2000, the minimum nominal cover for reinforcement in R.C.C. footings is 50 mm.
Conclusion on Minimum Cover for Footings
The minimum nominal cover for reinforcement in R.C.C. footings, as per the Indian Standard code IS: 456-2000, is clearly specified as 50 mm. This larger cover helps protect the steel from moisture and other harmful substances present in the soil, ensuring the long-term performance and durability of the footing.
| Structural Member | Minimum Nominal Cover | Remarks |
|---|---|---|
| Slabs | 20 mm (or clear size of bar) | Varies with exposure condition (Table 16) |
| Beams | 25 mm (or clear size of bar) | Varies with exposure condition (Table 16) |
| Columns | 40 mm (or clear size of bar) | Varies with exposure condition (Table 16) |
| Footings | 50 mm | Specific requirement for footings |
| Exposed to severe conditions (general) | 45 mm | As per Table 16, but footing has specific rule |
Revision Table: Key Nominal Cover Requirements
| Structural Element | Minimum Cover (IS: 456-2000) |
|---|---|
| Slabs | 20 mm |
| Beams | 25 mm |
| Columns | 40 mm |
| Footings | 50 mm |
Additional Information on Concrete Nominal Cover
Nominal cover is a critical parameter in reinforced concrete design. Its primary functions include:
- Corrosion Protection: Concrete provides an alkaline environment that passivates the steel reinforcement, preventing rust. The cover acts as a barrier against moisture, chlorides, sulfates, and carbon dioxide which can break down this passivating layer and cause corrosion.
- Fire Resistance: Adequate cover delays the heating of the reinforcement during a fire, maintaining the structural integrity for a longer period.
- Bond Strength: The concrete cover helps in the proper transfer of stress between the concrete and the steel reinforcement.
- Physical Protection: It protects the reinforcement from physical damage.
The required nominal cover depends on several factors:
- Exposure Condition: As classified in IS: 456-2000 (Mild, Moderate, Severe, Very Severe, Extreme), conditions like proximity to coast, contact with soil, or exposure to aggressive chemicals influence the required cover. Footings are usually in contact with soil, often requiring significant cover.
- Durability Requirements: The intended service life and the environmental conditions determine the necessary level of protection.
- Fire Resistance Requirements: Building codes specify minimum covers for different levels of fire resistance.
- Size of Reinforcement: The cover should also be sufficient to allow for proper compaction of concrete around the bars and ensure adequate bond. IS: 456 specifies that the nominal cover should not be less than the diameter of the bar, but the minimum values given in Table 16 and specific clauses like for footings usually govern.
For footings, the 50 mm cover requirement is a specific mandate in the code, overriding the general exposure condition requirements that might apply to other elements like beams or columns placed in similar severe environments.
Important Questions from Footings
For a number of columns constructed in a row, the type of foundation provided is
If p is the net upward pressure on a square footing of side b for a square column of side a, the maximum bending moment is given by
A. \({\rm{B}}.{\rm{M}} = \frac{{{\rm{pb}}\left( {{\rm{c}} - {\rm{a}}} \right)}}{4}\)
B. \({\rm{B}}.{\rm{M}} = \frac{{{\rm{pb}}{{\left( {{\rm{b}} - {\rm{a}}} \right)}^2}}}{4}\)
C. \({\rm{B}}.{\rm{M}} = \frac{{{\rm{pb}}{{\left( {{\rm{b}} - {\rm{a}}} \right)}^2}}}{8}\)
D. \({\rm{B}}.{\rm{M}} = \frac{{{\rm{pb}}\left( {{\rm{b}} + {\rm{a}}} \right)}}{8}\)
_______ is provided to support an individual column. It is circular, square or rectangular slab of uniform thickness Sometimes it is stepped or haunched to spread the load over a large area.
For a proposed building, raft foundation, isolated footings and combined footings are being considered. These foundations are to be listed in the decreasing order of preference in terms of performance. Which one of the following is the correct order of listing?
Which is a method of underwater concreting?
